Electrophotographic technology is used in a wide variety of fields including a copier, various kinds of printers and the like because it can quickly provide high quality photographic images. A photoreceptor using an organic photoconductive material which has advantages of no pollution, easy film forming, easy production, and the like is used for the photoreceptor which serves as a core of the electrophotographic technology.
As the photoreceptor using the organic photoconductive material, two types are known. One is a so called dispersion type photoreceptor using a fine powder photoconductive material which is dispersed in a binder resin. The other is a so called lamination type photoreceptor into which a charge generation layer and a charge transport layer are laminated. Particularly, the lamination type photoreceptor is the mainstream of the photoreceptor, is enthusiastically developed, and is put into practice, because it allows a highly sensitive photoreceptor to be obtained by combining the charge generation material and the charge transport material which respectively have a high efficiency of generating and transporting electric charges, because it offers such a wide selection of materials as to allow the obtaining of safe and secure materials, because it allows a photosensitive layer to be easily formed by means of application thereof, and also because it provides high productivity and advantage in cost reduction.
The electrophotographic photoreceptor is repeatedly used in an electrophotographic process, that is, in cycles of charging, exposing, developing, transporting, cleaning, and charge removal. Thus, it is subjected to such various stresses to be deteriorated during this cycle. These deteriorations include chemical and electrical ones such as, for example, chemical damages on the photosensitive layer caused by strong oxidizing ozone and NOx generated from a corona charger which is usually used as a charging device, and the decomposition of a composition in the photosensitive layer due to the flow of the carrier (electric current) generated in the photosensitive layer by exposing the image, the charge removal light, or the external light. Furthermore, there are mechanical deteriorations including the occurrence of wear and scratches on the surface of the photosensitive layer, and the peeling off of the film, due to the sliding abrasions of a cleaning blade and a magnetic blush therewith, and due to its contact with a developing agent and a paper. In particular, the damages generated on the surface of the photosensitive layer tend to appear on the image, and thereby directly diminish an image quality. Thus, they are the major factors which limit the service lifetime of the photoreceptor.
In the case of a general photoreceptor which is not provided with a functional layer such as a surface protecting layer, the photosensitive layer is subjected to these loads. The photosensitive layer is usually constituted of a binder resin and a photoconductive material. Its strength is substantially influenced by the binder resin. However, the amount of doped photoconductive material is considerably so large that the photosensitive layer has not been allowed to have the sufficient mechanical strength. Moreover, the increasing need for a high speed printing requires a material corresponding to a higher speed electrophotographic process. In this case, the photoreceptor must have a better quick response because of the need for shortening a period of time from exposure to development, as well as high sensitivity and a long service lifetime.
Each of the layers constituting the electrophotographic photoreceptor is usually formed by coating with a coating liquid containing a photoconductive material and a binder resin on a base support by means of dip-coating, spray coating, nozzle coating, bar coating, roll coating, blade coating. As these methods of forming a layer, well known methods are employed in which a coating solution obtained by dissolving the materials to be contained in the layer in a solvent is applied. For many processes, a coating solution is previously prepared, and is then stored.
As the binder resin of the photosensitive layer, thermoplastic resin and various kinds of thermosetting resins, including vinyl polymers such as poly(methyl methacrylate), polystyrene, and poly(vinyl chloride), and copolymers thereof, polycarbonate, polyester, polysulfone, phenoxy, epoxy, silicon resin. Among a large number of binder resins, the polycarbonate resin has relatively excellent performance. Thus, various kinds of polycarbonate resins have so far been developed and thus put into practice (refer to Patent Documents 1 to 4).
On the other hand, it has been reported that the electrophotographic photoreceptor, in which a polyarylate resin commercially available under the trade name of “U-polymer” is used as a binder, has a further improved sensitivity, compared to one using the polycarbonate (refer to Patent Document 5). Moreover, it has been reported that the use of the polyarylate resin, in which a bivalent phenol component having a particular structure as a binder resin improves the stability of a coating solution used when producing an electrophotographic photoreceptor, and in which the component further improves mechanical strength and wear resistance of the electrophotographic photoreceptor (refer to Patent Documents 6 and 7).
Patent Document 1: Japanese Laid-open Patent Application No. 50-098332
Patent Document 2: Japanese Laid-open Patent Application No. 59-071057
Patent Document 3: Japanese Laid-open Patent Application No. 59-184251
Patent Document 4: Japanese Laid-open Patent Application No. 05-021478
Patent Document 5: Japanese Laid-open Patent Application No. 56-135844
Patent Document 6: Japanese Laid-open Patent Application No. 03-006567
Patent Document 7: Japanese Laid-open Patent Application No. 10-288845